Public health authorities are investigating two highly unusual cases of a previously unknown strain of swine flu that was found in the San Diego area late last month. The cases occurred almost simultaneously in children who had no contact with pigs or each other, a scenario that raised the possibility that the illnesses may be the sign of an emerging pandemic strain of influenza.

More than 50 scientists and epidemiologists at the Centers for Disease Control and Prevention in Atlanta are studying the strain, and dozens of public health investigators in Southern California are looking for more cases among the those who had contact with the children. “While we have a low index of suspicion that this is a pandemic, we’re being very careful in our investigation to rule out every possibility,” said Lyn Finelli, an epidemiologist in the CDC’s influenza division.

Neither of the children — a 10-year-old boy in San Diego County and a 9-year-old girl in Imperial County, just to the east — was seriously ill. The cases were detected because both children were treated at clinics that took nose or throat swabs looking for influenza and passed the samples on to health department labs when they could not identify the strains.
Both children have recovered. The boy, however, took an airplane trip to Texas with his younger brother while at the tail end of his illness before it was known that he had an unusual strain of flu. Health officials in the Dallas area are looking for cases there, as well as among airline employees who assisted the two children, who traveled as “unaccompanied minors.”

Public health officials in the two California counties — both of which border Mexico — are urging physicians and hospitals to look carefully for cases of flu and report any to local health departments.
The Imperial County girl fell ill on March 28 with cough and a high fever, and the San Diego boy came down with similar symptoms, as well as vomiting, two days later. People in both households became sick before and after the children did, although health officials have not determined whether they also had swine flu.

Munday said his department has drawn blood from more than 20 people to be tested for antibodies to the swine flu strain. The antibodies would be a sign that those people were infected even if they never had symptoms. He said some had traveled into Mexico recently but would not describe them further. “As of yet, we have not been able to come up with any explanation of why anyone would have swine flu,” he said.

Molecular analysis of the virus suggests that it is the product of a rare event called a “gene reassortment.” In a reassortment, two distinct strains of virus infect the same cell. The viruses take over the cell’s genetic machinery to make copies of themselves, mingling the genes of the two strains to create a new, essentially hybrid, strain. Six of the eight genes in the new strain are from the North American lineage of swine flu, but two are from the Eurasian lineage. The reassortment probably occurred in a pig sometime in the past decade. Both sets of genes are slightly different from those of their original lineage — a sign that time has passed. However, it is unlikely that they have been in humans very long.